Cancer cells exhibit abnormalities in cell-cycle control and in differentiation. This is due, at least partly, to the fact that most cancer cells harbor mutations that compromise the function of the retinoblastoma protein (pRB.) The best understood targets of pRB are members of the E2F cell-cycle regulatory transcription factor family. E2F, unfettered by pRB, can induce cellular proliferation but can also induce apoptosis. E2F is neutralized in S-phase by cyclin NCdk2. Short peptides ('RXL peptides') that block the interaction of cyclin A/cdk2 with E2F selectively kill transformed cells, perhaps due to the high levels of E2F that typify cancer cells compared to normal cells. Specific aim 1 will be to confirm or refute the hypothesis that RXL peptides kill cells by preventing the neutralization of E2F by cyclin A/cdk2. Control of the G1/S transition by pRB is linked to its ability to repress E2F-responsive promoters, whereas control of differentiation is not. Two additional pRB interactors, EID1 and RBP2, might be linked to differentiation control. EID1 inhibits p300/CBP histone acetylase activity, blocks differentiation, and is rapidly polyubiquitinated as cell exit the cell-cycle. Understanding the control of EID1 turnover will be the focus of Specific aim 2. Preliminary data suggests a role of pRB and RBP2 in differentiation-dependent chromatin remodeling. The functional significance of pRB/RBP2 interactions will be addressed in Specific aim 3 and specific aim 4 will ask whether EID1 and/or RBP2 contribute to the developmental abnormalities observed in RB-/- mice.